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A reliable description of surfaces structures in a reactive environment is crucial to understand materials functions. We present a first-principles theory of replica-exchange grand-canonical-ensemble molecular dynamics (REGC-MD) and apply it to evaluate phase equilibria of surfaces in reactive gas-phase environment. We identify the different surface phases and locate phase boundaries including triple and critical points. The approach is demonstrated by addressing open questions for the Si(100) surface in contact with a hydrogen atmosphere. In the range from 300 to 1 000 K, we find 25 distinct thermodynamically stable surface phases, for which we also provide microscopic descriptions. Most of the identified phases, including few order-disorder phase transitions, have not yet been observed experimentally. The REGC-MD-derived phase diagram shows significant, qualitative differences to the description by the state-of-the-art "ab initio atomistic thermodynamics" approach.

Searching for high-energy-density materials is of great interest in scientific research and for industrial applications. Using an unbiased structure prediction method and first-principles calculations, we investigated the phase stability of LiBN2 from 0 to100 GPa. Two new structures with space groups P421m and Pnma were discovered. The theoretical calculations revealed that Pnma LiBN2 is stable with respect to a mixture of 1/3Li3N, BN, and 1/3N2 above 22 GPa. The electronic band structure revealed that Pnma LiBN2 has an indirect band gap of 2.3 eV, which shows a nonmetallic feature. The Pnma phase has a high calculated bulk modulus and shear modulus, indicating its incompressible nature. The microscopic mechanism of the structural deformation was demonstrated by ideal tensile shear strength calculations. It is worth mentioning that Pnma LiBN2 is dynamically stable under ambient conditions. The decomposition of this phase is exothermic, releasing an energy of approximately 1.23 kJ/g at the PBE level. The ...

Using a diamond-anvil cell (DAC), we conducted in situ angle dispersive X-ray diffraction (ADXRD) up to 47.6 GPa at room temperature to evaluate the structural stability of α-As2Te3. A reversible structural transition was observed at between 17.4 and 36.3 GPa. The high-pressure structure is solved by a γ-Bi2Te3-type monoclinic phase (C2/c, γ-As2Te3). Reliable structural analyses, such as Rietveld refinement of the powder x-ray data, were provided, which show that the previously reported high-pressure β-As2Te3 phase is not existed. It is worth mentioning that twice pressure-induced electron phase transitions were revealed in α-As2Te3 at about 3 and 6 GPa, which cause the striking fluctuation in thermoelectric property at that pressures.

The structural, electronic, and mechanical properties of the Fe2B with the I4/mcm structure were investigated using first principles calculations. The dynamical stability and mechanical stability of the I4/mcm phase were confirmed, according to the calculated phonon dispersion and elastic constant. The theoretical band structure and density of states showed I4/mcm Fe2B to be metallic. The microscopic mechanism of the structural deformation of I4/mcm Fe2B was examined by calculating ideal strength values at large levels of strain. Our theoretical results demonstrated the failure mode in I4/mcm Fe2B to be dominated by shear, with the lowest pure shear peak stress calculated to be 20.0 GPa.

Simple molecular solids are expected to undergo structural phase transitions to form frameworks or polymeric solids under high pressure. The high-pressure structures of N2O have long attracted considerable attention. We combine the CALYPSO searching method with density functional theory to investigate the phase stabilities and structural changes of N2O at high pressure. We find two metallic structures of N2O which may be observed in high-pressure experiments. The current calculations reveal that the C2/m is the most stable structure over a pressure range of 177–194 GPa, and the other C2/c is metastable and only 10 meV per atom higher in energy than the C2/m structure at 180 GPa. At higher pressure, the metallic C2/m phase transforms into an insulating phase with space group of P21/m.

We have extensively investigated the crystal structures of N2CO under high pressure using the swarm structure searching technique in combination with density functional theory. Three single-bonded 3-dimensional structures with space groups of P43, P43212, and P21212 are discovered. We show that the P43 phase is the most stable structure compared with N2 and CO above 35.6 GPa. The distribution of the excess electrons on the N and O atoms does not form bonds but forms a stable lone-pair state. The strong covalent bonds together with lone-pair states are the driving force for the high bulk shear modulus of N2CO. The P43 structure may be used as an advanced energetic material with an energy density of approximately 4.6 kJ g–1, which is a little higher than the modern explosive TNT (4.2 kJ g–1).

The high-symmetry cubic cesium chloride (CsCl) structure with a space group of Pm3¯m (Z = 1) is one of the prototypical AB-type compounds, which is shared with cesium halides and many binary metallic alloys. The study of high-pressure evolution of the CsCl phase is of fundamental importance in helping to understand the structural sequence and principles of crystallography. Here, we have systematically investigated the high-pressure structural transition of cesium halides up to 200 GPa using an effective CALYPSO algorithm. Strikingly, we have predicted several thermodynamically favored high-pressure phases for cesium chloride and cesium bromide (CsBr). Further electronic calculations indicate that CsCl and CsBr become metallic via band-gap closure at strong compression. The current predictions have broad implications for other AB-type compounds that likely harbor similar novel high-pressure behavior.

There is an urgent need for the development in the field of the magnetism of topological insulators, owing to the necessity for the realization of the quantum anomalous Hall effect. Herein, we discuss experimentally fabricated nanostructured hierarchical architectures of the topological insulator Bi2Te3 without the introduction of any exotic magnetic dopants, in which intriguing room-temperature ferromagnetism was identified. First-principles calculations demonstrated that the intrinsic point defect with respect to the antisite Te site is responsible for the creation of a magnetic moment. Such a mechanism, which is different from that of a vacancy defect, provides new insights into the origins of magnetism. Our findings may pave the way for developing future Bi2Te3-based dissipationless spintronics and fault-tolerant quantum computation.

High pressure can fundamentally alter the electronic structure of elemental metals, leading to the unexpected formation of intermetallics with unusual structural features. In the present study, the phase stabilities and structural changes of Na-Fe intermetallics under pressure were studied using unbiased structure searching methods, combined with density functional theory calculations. Two intermetallics with stoichiometries Na3Fe and Na4Fe are found to be thermodynamically stable at pressures above 120 and 155 GPa, respectively. An interesting structural feature is that both have form a host-guest-like structure with Na sublattices constructed from small and large polygons similar to the host framework of the self-hosting incommensurate phases observed in Group I and II elements. Apart from the one-dimensional (1D) Fe chains running through the large channels, more interestingly, electrides are found to localize in the small channels between the layers. Electron topological analysis shows secondary bonding interactions between the Fe atoms and the interstitial electrides help to stabilize these structures.

We present a study on NaBH 4 under high pressure by using the ab initio pseudopotential plane-wave method. The calculations show that a structural phase transition takes place at 10.91 GPa from the β-NaBH 4 structure (tetragonal-P42 1 c) to the γ-NaBH 4 structure (orthorhombic-Pnma). Band-structure calculation reveals that the two phases show an insulator behaviour with a finite energy gap. There is no pressure-induced softening behaviour from our calculated phonon dispersion curves near the phase transition pressure. We also characterize charge transfer and Mulliken population analyses of these structures. The Mulliken population analyses indicate that the β-NaBH 4 phase is expected to be the most promising candidate for hydrogen storage.

We report a new member to the family of tungsten nitrides, WN6, predicted from the structure search. Ground-state convex hull calculation reveals that crystalline WN6 is thermodynamically stable at pressures above 16 GPa, but remains dynamically stable at ambient conditions. The predicted high-pressure WN6 structure contains chaired rings isoelectronic to cyclo-hexasulfur (S6), which is unprecedented in nitrogen. In the unit all nitrogen atoms are singly bonded and therefore contain a high energy density. By means of efficiently packing the covalent-bonded species, WN6 is estimated to have extremely high Vickers hardness greater than 40 GPa at ambient conditions, placing it as one of the hardest materials. The present results reveal that WN6 may be used as a superhard material but simultaneously maintaining other desirable properties, which represents an interesting example of multifunctional materials.

First-principles calculations have been performed on NaAlH 4 using the generalized gradient approximation pseudopotential method. The predicted β-NaAlH 4 (α-LiAlH 4 -type) structure is energetically more favorable than α-NaAlH 4 for pressures over 15.9 GPa, which is apparently correlated with the experimental transition pressure 14 GPa. This transition is identified as first-order in nature with volume contractions of 1.8%. There is no pressure-induced softening behavior from our calculated phonon dispersion curves near the phase transition pressure. Based on the Mulliken population analysis, the β-NaAlH 4 structure is expected to be the most promising candidate for hydrogen storage.

Caterpillar-like Bi2Te3 hierarchical architectures are synthesized from elemental Te in trioctylphosphine and Bi(OAc)3 in the presence of oleic acid and 1-octadecene (autoclave, 200 °C, 4 h).

We reported a developed methodology to design superhard materials for given chemical systems under external conditions (here, pressure). The new approach is based on the CALYPSO algorithm and requires only the chemical compositions to predict the hardness vs. energy map, from which the energetically preferable superhard structures are readily accessible. In contrast to the traditional ground state structure prediction method where the total energy was solely used as the fitness function, here we adopted hardness as the fitness function in combination with the first-principles calculation to construct the hardness vs. energy map by seeking a proper balance between hardness and energy for a better mechanical description of given chemical systems. To allow a universal calculation on the hardness for the predicted structure, we have improved the earlier hardness model based on bond strength by applying the Laplacian matrix to account for the highly anisotropic and molecular systems. We benchmarked our approach in typical superhard systems, such as elemental carbon, binary B-N, and ternary B-C-N compounds. Nearly all the experimentally known and most of the earlier theoretical superhard structures have been successfully reproduced. The results suggested that our approach is reliable and can be widely applied into design of new superhard materials.

Using density-functional-theory calculations, a monoclinic metallic post-ζ phase (space group C2/c) is predicted at 215 GPa. The calculated phonon dispersion curves suggest that this structure is stable at least up to 310 GPa. Oxygen remains a molecular crystal and there is no dissociation in the related pressure range. Moreover, it is found that the phase transition from ζ to post-ζ phase is attributed to phonon softening. The significant change in the optical properties can be used to identify the phase transition.